This invention relates to a hip prosthesis to be fixed to the femoral joint, and more particularly, to a femoral stem prosthesis for insertion into the intramedullary canal of the femur.
The use of hip joint prostheses having ball-shaped head portions and stems which are fixed within the intramedullary canal is well-known. Typically, the prosthesis is secured within the canal with the aid of special cement. See, for example, U.S. Pat. No. 4,404,692. It is also known to secure hip joint prostheses by various mechanical means including screws (U.S. Pat. No. 4,101,985), expandable stems (U.S. Pat. No. 4,520,511), expandable stem segments (U.S. Pat. No. 3,846,846) and extendable leg anchors (U.S. Pat. No. 3,024,785); see also U.S. Pat. No. 2,685,877. Nevertheless, loosening of the femoral component of the total hip replacement, caused by rotary or torsion forces about the upper end of the femoral shaft, continues to be a problem. Once the prosthesis has been loosened, it must be surgically reset.
According to the present invention, a femoral stem prosthesis is provided having an elongated stem portion for insertion into the intramedullary canal of the femur. The elongated stem portion has upper and lower ends, a longitudinal axis, and a threaded bore extending generally along the longitudinal axis from the upper end of the stem portion to a point within an elongated aperture located generally in the upper half of the stem portion. The stem is also provided with a plurality of retaining notches formed in laterally aligned pairs along opposite sides of the stem below the elongated aperture. Anchoring means are provided for firmly attaching the prosthesis to the surrounding walls of the intramedullary canal in the form of one or more elongated resilient spring steel strips carried by the stem portion. The stem portion also carries an adjustable actuator screw in the threaded bore. The adjustable actuator screw at its lower end engages a plunger slidably received in the bore. The plunger is formed with an enlarged retaining collar at an end remote from the screw engagement, the collar being free to move axially within the elongated aperture formed beneath the threaded bore. Each of the elongated resilient spring steel strips have a first end received in the retaining collar and a second end received in one of the retaining notches formed in the stem portion. The construction is such that upon rotation of the adjustable actuator screw to effect downward movement of the plunger, pressure is applied through the retaining collar on the elongated resilient spring strips to cause the strips to bow outwardly in a direction perpendicular to the longitudinal axis of the stem. As the adjustable actuator screw is rotated further, the spring steel strips continue to expand outwardly until they firmly engage the walls of the intramedullary canal. Additional apertures may be formed in the stem portion to provide ample room for ingrowth of bone tissue in and around the spring steel strips and the prosthesis itself to thereby permit the formation of a large, strong, bony resistant mass to further counteract the torsion forces which would otherwise tend to loosen the prosthesis.
According to the present invention, at least two and as many as three laterally aligned pairs of retaining notches are formed along the length of the stem portion to accommodate spring strips of different lengths. In this way, the length and location of the spring strips, and therefore the expansion capability of the device, may be selected as required. In addition, the number of spring strips may be varied. In a preferred embodiment, at least one pair of spring strips of equal length are utilized. In their expanded state, the spring strips form a generally elliptical shape which provides a particularly strong anchoring configuration. it is to be understood that as many as three pair of spring strips may be used concurrently.
The upper end of the stem portion has a ball-shaped head of a conventional type.
In an alternative embodiment of the invention, a modified actuator screw is used which eliminates the need for the spring retaining notches formed in the stem. The screw is rotatably received within a smooth bore formed in the stem. The lower portion of the screw extends into a slot formed in the stem and is threaded in order to receive a modified nut which is held against rotation in the slot but is free to move axially therein. In this embodiment, the spring strips are held between a flange formed at the upper end of the stem and an upwardly facing recess formed in the axially moveable nut. Rotation of the actuator screw moves the nut upwardly causing the spring strips to bow outwardly in a manner similar to that described above.